Communication is key bro. I handed my phone over to my gf just to look at a photo, and all of a sudden she was starring into my phone like she was inspecting and I couldn't see what she was doing. I told her I don't like it when people look at my phone and I can't see what they are looking at, I want to have control of the situation and the way that I present myself to her. In the future if/when we get married, she will have full access to my phone, but we are not there yet. It just feels invasive to me, like reading somebody's private diary.
She asked for examples of what I could possibly not want her to see, I said maybe an unflattering photo, I don't know, could be anything.
She understood, if she didn't it would be a red flag.